As a scratch builder I am in the process of drafting the rib parts to build the form blocks.  Each rib part has been drafted using AutoCAD Version 2010.  Each individual rib section draws out fairly well from the drawings but I started thinking about the matching of end rib compared to a nose and rear rib assembly.

 

The Horizontal Stabilizer is built of the End Rib 75-T-1-1 and the internal assembly is built up of 75-T-1-2 Nose Rib and 75-T-1-3 Rear Rib.  If I over lap the Nose Rib and the Rear Rib with the End Rib they are the same length from nose to trailing edge.  Looking at the assembly the Rear Rib is assembled between a spar and a doubler on both the front and back and the Nose rib is attached on the other side of the front spar.  This assembly adds the thickness of the spar and the doubler between them to the overall length of the assembly plus the thickness of the two additional flanges on the front and rear of the Rear Rib and the Nose Rib respectively.  The overall dimension will be several sheet thicknesses longer than the end rib.

 

The assembly is further complicated by the fact that the edges of each rib on the drawings are actually construction lines for the blank for that rib.  The actual dimension of each rib should grow by the thickness of the material as the material is bent over the edge of each blank adding two thicknesses to the length of each piece.  Obviously the drawings work but it appears that the Nose and Rear Ribs have not been adjusted for the thickness of the various parts being assembled between them.  Am I worrying about this unnecessarily?  I could go though and calculate the developed lengths of each piece and adjust the dimensions but I am surprised that Zenith hasn't done this.

 

Just starting this process myself, perhaps everyone has found that the tolerances of the individual pieces get swallowed up in the assembly? 

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Hello Joe, you have a valid point, and I was wondering the same. I am surmizing that the radius we put on the enge of the forms will correct the dimensional issue and any extra material folded over will just give a bigger footprint on the rivet line.

Joseph,

I am scratch building the 701 and have the rudder, stabilizer, elevator and both wing skeletons done. The left wing has the top and bottom skin clecoed on and I guess the fuel tank will be next.  The material you are talking about is .016 and .025, (on the 701 anyway) about like copy paper and card stock, not a big deal. Just build the form blocks as the plans tell you and everything will work out. Don't over think it, a millimeter or two isn't going to make much difference in those areas and from 10 feet away you won't see it. I always take "hard" measurements when cutting skins and such and it pretty much comes out just like the plans says it should.

Don't make it more difficult that it is.

Bob

P.S. You may look around locally for material and save some shipping. I found a place called Metal Supermarket that carries 6061-T6 and they have locations all over.

Joseph - Robert has a point. As another plans builder I'm familiar with what you're eluding to. Just keep your total length of the individual part within that mm or 2 as Robert says and you'll be fine. Your biggest challenge on that part (horizontal) will be fitting the skin to the skeleton without twisting the whole thing...... There will be other times to sweat. I don't think your at that point right now. Best of luck!

Chris

Thank each of you for the positive thoughts.  Since there have been so many building off of the plans I figured that the milimeters would disappear in the skeeme of things.  I did give Mark Townsend a call and talked about the numbers.  He says that he hasn't built a 750 from scratch yet but that you just tuck and squeeze and it all comes together.  It wouldn't take much to adjust the end ribs and tweak things to line things up.  May make it easier to keep things from twisting if we weren't trying to stretch the skin over the uneven skeleton so I will probably go ahead and make the adjustments.

 

I will report how it all works out.

Even with perfect measurements the possibilty of twisting is very real. But I believe you have a handle on it. Best of luck.

Chris

Hi Joseph,

I am plans building a 701, but many of the issues cross pollinate with the 750. In my experience CH does not worry too much about perfection, his idea is that form follows function. During my build I have had to 'persuade' the odd part to fit by careful use of bending pliers etc on the flange (obviously within reason and mindful of edge distances). Just to reiterate what others have said the fitting the skin without twist is the critical challenge. I built mine flat then decided to update to the thicker .063 attachments of more recent plans. I thought that grip pinning the skin into the original holes would automatically reproduce a flat part, wrong, I ended up with a twist and had to re jig it using strapping to a flat table. It just shows how important the strapping down to a flat bench is. All the best with your build.

Regards, Andrew.

See some pics on my profile:   http://www.zenith.aero/photo/albums/063-horiz-tail-brackets

 

Interesting observation I am just starting out and a scratch builder myself. I will be following your progress and hopefully share info with you as I progress.

I've been using Microstation the past two years and I'm still clumbsy with it. I really miss the ease of autocad. I have 2002 version on my laptop. What sort of prices are you getting for your watercuts? Would like to talk with you.

Sincerly Marty

Marty

I am not doing anything with the drawings other than generating templates for my blanks.  I wish I had access to water cutting but that is not the case.  You must have me confused with another builder.  I know that there are a agroup of six guys in Canada that are building six 750's together and they are water cutting a bunch of their parts but they have quantity on their side.

Joe B

Marty,

The machine shop that we used charges $250/hr for their water jet. It took 88 minutes to cut the parts that were 0.090" and thicker for the 6 airplanes.

Joe

gee thanks a lot great info. I just spent My Mardi Gras Day drawing up the .025 parts I wanted to have watercut. I just emailed it to my local watercutter, I now know if he will be over pricing me or not.

Thank you very much

Marty

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